1. Field of the Invention
This invention is concerned with the field of apparatuses and methods for applying coatings to paper or other similar sheet materials, and in particular, to improvements in such wherein the thickness of the coating is monitored and regulated while being applied.
2. Background Art
In the process of making paper, it is often desirable to coat the paper with any of a wide variety of materials. For example, paper is often coated with a sizing material which is a paste-like substance, usually containing silica, for smoothing out the surface of the paper by filling in the pores that are formed between the wood pulp fibers that make up the paper. Other coatings or precoatings are applied to paper and other sheet materials during the paper-making process. For example, on specialty papers such as reprographic grades or oil/grease-proof packaging, where uniform film forming or solvent holdout are of importance, combinations of coatings are used. As another example, adhesive materials are applied where paper is to be used as tape or labels. As used herein, "coating" includes any type of coating or sizing applied to paper or other sheet materials by the type of apparatuses discussed.
Such coatings are usually applied to paper as a part of the paper making process in a paper mill where the paper may be disposed in rolls that are on the order of 25 feet or more in the cross direction. Uniformity of coating thickness is often necessary or desirable for various reasons. For example, the printability of paper is improved by the uniform application of sizing. The coating thickness, in some cases, must be controlled to within microns or less.
Coating apparatuses or coaters come in a wide variety of configurations. One type comprises on one side a rotating backing drum and on the opposite side a metering element which may be a blade edge, both of which are oriented in the cross direction. The drum and the blade edge form a narrow slot through which the sheet of paper or other material to be coated, often termed the "web," passes. The blade presses the paper with the coating applied as it passes through the slot, removing excess coating. The web may pass either up or down through the slot depending on the configuration. The use of a long single blade rather than several shorter blades eliminates the lines that would be caused on the coated paper or other material by the slight discontinuity of adjacent blades where they meet one another.
It will be appreciated that the separation of the drum from the metering element is a critical factor in the application of such coatings. The drum is fabricated and installed to high tolerances. In order to try to control the thickness of the coatings applied to webs in such an apparatus, coaters provide a means of adjusting the position of the metering element before the process begins, usually at equally spaced intervals in the cross direction. Each adjustment usually covers an interval, known as a "slice," in the cross-direction. Each slice may be three inches or some other length. For example, in some coaters, a flexible steel blade is used as the metering element. The blade is held between blade jaws oriented in the cross-direction along a side of the blade opposite to the edge which forms the slot. Adjusting screws are disposed at selected equal intervals in the cross-direction, one per slice, and are disposed to press against the back of the blade between the blade jaws and the edge forming the slot. It will be appreciated that because the blade is a thin steel member, the blade may be slightly bent or flexed in the vicinity of an adjusting screw by adjusting the screw, thereby varying the width of the slot formed by the drum and the blade edge in that vicinity. Such adjustments of the adjusting screws are done prior to a coating procedure in order to obtain a slot of uniform width and a coating which is uniform to the extent possible. The adjusting screws can be adjusted to take into account factors that result in non-uniform coatings that are known before the coating process begins. In the case where a rod is used as the metering element, the rod may be held by aluminum supports with the adjusting screws positioned to press on the aluminum supports.
Even with coaters that include such screw adjustments at equally spaced intervals, it is still difficult to get even coatings on paper. Local variations in temperature and paper thickness, and possibly other factors, may be responsible for uneven coatings. Furthermore, while the adjustment screws may be set before a coating procedure is initiated, the adjusting screws are not adjusted during the coating procedure. Coaters may operate at speeds of 3000 feet per minute or more, and the adjusting screws are located in relatively inaccessible places, often rendering it highly impractical and unsafe, if not impossible, to manually adjust the adjusting screws during a coating procedure. The environment in which the adjusting screws is located, moreover, aside from allowing for little space, is highly unsuited to the use of precision mechanisms to permit the adjusting screws to be adjusted remotely.
It will be appreciated from the foregoing that the inability to control the separation of the metering element and the drum at a plurality of cross-directional positions during the coating procedure results in coatings which may be of less uniform thickness than would result in the case where such separation at a plurality of cross-directional positions could be adjusted while the coating procedure is taking place.